1. Field of the Invention
The present invention relates to a motortricycle with an oscillation mechanism, more particularly to an engine support structure and a vehicle body frame structure for supporting the engine.
2. Description of Background Art
As a motortricycle with an oscillation mechanism, those in which the engine can swing with the rear wheels have been known. For example, Japanese Patent Laid-open No. Sho 59-153674 and Japanese Patent Publication No. Hei 1-23356 have been known. The contents of Japanese Patent Laid-open No. Sho 59-153674 will be described below referring to FIG. 18, and the contents of Japanese Patent Publication No. Hei 1-23356 will be described below referring to FIG. 19.
A vehicle body frame for a vehicle, is known wherein a driving device support frame has a substantially box shape for seated type four-wheel buggy car. For example, Japanese Utility Model No. 2521705 has been known. The contents of Japanese Utility Model No. 2521705 will be described below referring to FIG. 20 and FIG. 21.
FIG. 18 shows a motortricycle in which a connection shaft 203 is mounted onto a rear portion of a front frame 201 through a bracket plate 202, a drum 204 is turnably mounted onto the connection shaft 203, a back tube 205 is attached to the drum 204, an engine unit 207 is mounted onto the back tube 205 through a support shaft 206, left and right rear wheels 208 are mounted onto rear portions of the engine unit 207, and a shock absorber 211 is disposed bridgingly between the engine unit 207 and the rear end of the back tube 205.
Both the engine unit 207 and the rear wheels 208 are vertically movable relative to the back tube 205. With this structure, there arises no problem in the case of a vehicle having a comparatively small displacement. However, in the case of high-speed running of a vehicle having an intermediate to large displacement, the weight under the spring 211 will be large. Therefore, in this situation, it is difficult for the rear wheels 208 to follow the surface of a very rough road on which the vehicle is running.
FIG. 19 shows a motortricycle 226 in which an oscillation joint 218 is vertically movably mounted onto a rear portion of a front vehicle body including a front wheel 215 through a pin 217, a rear frame 222 constituting an essential part of a rear vehicle body 221 is attached to a rear portion of the oscillation joint 218, Further, an engine 223 is mounted onto the rear frame 222, and left and right rear wheels 225 are mounted onto an output shaft of a transmission case 224 provided integrally with the engine 223.
In the motortricycle 226, since the engine 223 is mounted onto the rear frame 222 swingably mounted onto the front vehicle body 216, the under-spring weight will be large. Therefore, in the same manner as in the case of the motortricycle shown in FIG. 18, it is difficult for the rear wheels 225 to follow up to the ruggedness of the ground surface in the case of a vehicle having an intermediate to large displacement.
As shown in FIG. 20, a seat mount frame 231 L-shaped in side view is mounted onto a rear portion of a main frame 230, an engine support frame 232 L-shaped in side view is mounted onto the rear end of the main frame 230, the rear end of the seat mount frame 231 is attached to a front portion of the engine mount frame 232, and an intermediate portion of the engine support frame 232 is supported by a slant frame 233 extended rearwardly upwards from the rear end of the main frame 231. Further, a rear frame 234 roughly L-shaped in side view is attached to the rear end of the engine support frame 232, a lower frame 235 is arranged bridgingly between the front end of the rear frame 234 and a lower portion of the slant frame 233, and an engine 236, a non-stage speed change type belt converter transmission 237, and a differential gear 238 are surrounded by the main frame 230, the seat mount frame 231, the engine support frame 232, the rear frame 234, and the lower frame 235.
FIG. 21 shows the condition where the lower frame 235 roughly V-shaped in plan view is arranged bridgingly between left and right L-shaped pipes 241, 241 constituting the engine support frame 232, rear swing arms 242, 242 are swingably attached to the lower frame 235 through brackets 243, 243, and rear wheels 244, 244 are disposed on the end portion sides of the rear swing arms 242, 242.
In the above-mentioned prior art of FIG. 20 and FIG. 21, for example, the engine support frame 232 has a structure in which the L-shaped pipes 241, 241 disposed on the left and right sides are connected by a plurality of cross members, so that the weight thereof will be large although the rigidity thereof will be high. If a reduction in weight can be contrived while securing the rigidity for supporting the engine 236 and other power transmission mechanisms, the inertial weight of a rear portion of the vehicle body frame can be reduced.
In addition, when the L-shaped pipes 241, 241 are disposed on the left and right sides, the vehicle width is enlarged. For example, where the above buggy car is a vehicle having such a structure as to permit large oscillations to the left and right sides, the L-shaped pipes 241, 241 are liable to interfere with the rear swing arms 242, 242 upon oscillations to the left and right sides. Therefore, the rear swing arm 242 must, for example, be curved largely so as to obviate the interference with the L-shape pipe 241, so that the total length thereof is increased, resulting in an increase in weight.